An example of a large group of industrial processes that involve a number of different chemical reactions taking place simultaneously in large, expensive reactor vessels are those involved in the production of organic carboxylic acids particularly terephthalic acid (TPA). TPA is a major industrial chemical product, its principal application being the manufacture of various polymers, particularly polyethylene terephthalate (PET), which has widespread use in the production of synthetic fibers, fabrics and films, and particularly the production by blow molding of transparent food and drink containers. Owing to the quantities required the production of TPA is carried out in large installations in order to achieve economy of scale, the reaction vessels and associated equipment employed being correspondingly very large and expensive. The currently available processes involve a number of different reactions, and it is common to carry out at least some of the reactions simultaneously in a single vessel, in order to reduce the amount of the large, complex and expensive equipment required. The simultaneous performance of such reactions generally means that the conditions of temperature, pressure and concentration within the reactor vessel or vessels is a compromise between those that otherwise would be preferred for the different reactions.
Another result of this need to operate with large size installations (e.g. currently it is considered that a plant for TPA production must have a capacity of at least 100,000 tonnes per annum) is that it has become a major financial decision to begin construction of a new plant, or expansion of an existing plant, when current production has become, or is about to become, insufficient. When the new plant, or expansion, comes on stream the problem then arises that for a considerable period, which may be as long as several years, there is overproduction which depresses the available price and considerably extends the period required before both the new and the existing plants again become sufficiently profitable. The problem is exacerbated if two or more producers decide at about the same time that new production capacity is required, and each builds a plant. A corresponding situation arises when there is a down-turn in the economy necessitating a reduction in production. Producers are faced with the unwelcome choice of whether to keep a plant operating, resulting in overproduction and lowered price, or to shut down completely, itself usually an expensive process with attendant labor difficulties, and with resultant inadequate production. Again, the situation is exacerbated if two or more producers make the same decision at about the same time.
As stated in “Industrial Organic Chemistry-important Raw Materials and Intermediates” of Klaus Weissermel and Hans-Jürgen Arpe, 2nd edition published 1993 by Verlag Chemie, chemically the simplest method for producing PET is the polycondensation via direct esterification of TPA and ethylene glycol, but is only possible if the TPA is of the so-called “fiber grade” of purity. For example, in order to be suitable for the preparation of polyester fibers the TPA must be substantially free of contaminants which lower the melting point of the polyester and/or cause the formation of color bodies. Some impurities in crude TPA, such as 4-carboxybenzaldehyde, are color-forming precursors and can also act as chain terminators preventing the production of polymers of sufficiently high molecular weight. These impurities are difficult to remove from crude TPA, owing to the very low solubility of TPA in polar solvents such as water or glacial acetic acid, even at elevated temperatures and pressures. An early process to obtain TPA of high purity involved converting the crude TPA to the dimethylester, which could economically be brought to fiber grade by crystallization and/or distillation. Older less energy-efficient PET processes employing DMT have largely been superceded by more modern facilities that use the purified TPA in a direct esterification, rather than the lower-yielding, slower-reacting DMT transesterification processes.
A common first stage in the production of TPA is the liquid phase oxidation of p-xylene in large vessels in which the contents are agitated, typically at temperatures in the range 100–200° C., sometimes in the range 140–170° C., and at pressures in the range 4–20 bar, sometimes 4–8 bar, and with residence times in the reactor vessels of up to 22 hours. The oxidation will stop at the stage where p-toluic acid (one methyl group, one carboxyl acid group) is produced unless special steps are taken to convert the second methyl group to the carboxyl acid. There are three main ways in which this has been achieved; many others have been used and proposed but they are of lesser commercial importance.
In a first method the carboxyl group of the p-toluic acid is esterified with methanol, whereupon subsequently the second methyl group can be oxidized. It is usual to use the methanol as a solvent during the oxidation, so that the esterification occurs simultaneously with the oxidation in a single reaction vessel. The method yields dimethyl terephthalate which, after separation and purification to fiber grade, is hydrolyzed readily to the TPA.
In a second method, now usually referred to as the Amoco process, and the predominant process for the manufacture of TPA, a metal salt catalyst, usually a mixture of manganese and cobalt acetate in 95% acetic acid, is employed, together with a co-catalyst or promoter such as ammonium bromide and tetrabromoethane, or a mixture of manganese and cobalt bromides, the bromine ion functioning as a regenerative source of free radicals that continue the oxidation. Temperatures of 190–205° C. and pressures of 15–30 bar are employed; the reaction vessels are extremely large and, together with the associated equipment, must be lined or made with materials able to withstand the corrosive attack of the bromine ions, so that they are correspondingly initially expensive and costly to maintain. Typically titanium or Hastelloy C™ alloys are employed. The TPA obtained must be purified and this is done by dissolving it under pressure at 225–275° C. in water and hydrogenating the solution in the presence of a platinum/charcoal catalyst, whereby the chief impurity 4-carboxylbenzaldehyde is hydrogenated to p-toluic acid. The pure TPA crystallizes out upon cooling the solution. The process is also suitable for oxidizing other methylbenzenes and methylnaphthalenes to aromatic carboxylic acids, e.g. toluene to benzoic acid; m-xylene to isophthalic acid; pseudocumene to trimellitic anhydride; mesitylene to trimesic acid; and 1,4-dimethylnaphthalene to naphthalene-1,4-dicarboxylic acid.
The third method, which is no longer in commercial use, is a co-oxidation process in which the principal reactions are all carried out together in a single vessel. Thus, the oxidation is carried out in the presence of an auxiliary substance which is simultaneously oxidized, and that is capable of supplying hydroperoxides that will complete the oxidation process. In one typical process p-xylene, together with paraldehyde and cobalt acetate in acetic acid solution are introduced at the head of a tall (e.g. 30 meters) bubble column, while air is introduced at the bottom, a typical temperature being 100–140° C. and a typical pressure being 30 bar. The bubbles ascending in the column are relied upon to produce the necessary interaction and chemical reaction between the components. The TPA is removed as a suspension in the acetic acid, separated and purified.